This invention relates to thermal insulation materials having high fire resistance and low thermal conductivity, and is particularly concerned with resin compositions and composites, particularly polyimide resins and composites formed therewith, incorporating substances in the resin to substantially increase fire resistance, and which are particularly applicable as an acoustic panel-fire wall structure in aircraft.
In order to reduce aircraft weight, thereby saving fuel, it is desirable to use graphite-resin composites in as many areas as possible. One location for use of such composites is the engine nacelle; and one application in that area is the acoustic panel. However, this area is also in the hot zone. Thus, in addition to serving as an acoustic panel, it must also serve as a fire barrier between the hot zone of the engine and the outer region, which contains fuel lines, hydraulic lines, electrical components, etc. Furthermore, there is an FAA requirement that any fire barrier used in the engine nacelle must be capable of withstanding a 2,000.degree. F. flame for 15 minutes.
Thus, using graphite-polyimide as the composite, a structural component is available that has good strength, is light weight and has some fire resistance, in that the polyimide will not burn at low flame temperatures. However, at 2,000.degree. F. flame conditions, the polyimide will burn and decompose to form a relatively stable char. This char, however, will allow heat to pass through to the backside and decompose the resin. This, in turn, creates a hazard due to the possibility of the decomposition products igniting, thereby starting a fire, even though the parent polymer (polyimide) will not readily burn. Therefore, a need exists for a non-burning resin that has good stability and low thermal conductivity.
Various ways have been suggested to effect protection of the acoustic panel-fire wall structure. One approach is to use a titanium face shield over the panel, thereby preventing a fire from getting through. Another approach is to use a high silica glass fabric face shield, or a silicone rubber impregnated high silica glass fabric. In the first instance, use of titanium does not help in weight reduction over use of graphite-polyimide composite. In the second approach, the high silica glass fabric or silicone-rubber impregnated high silica glass fabric also offers some protection, but it again necessitates the use of an extra barrier over the acoustical panel.
It is therefore necessary that some substance be incorporated into the polyimide that will give direct protection to the polyimide by being non-burning, have a low thermal conductivity, and be a char stabilizer. As mentioned above, there may be employed either a face shield of titanium, glass fabric or silicone-rubber impregnated glass fabric. Another alternative is to incorporate hydrated aluminum oxide directly into the polyimide for fire protection. Each of these methods is satisfactory, but suffers from limitations, such as increase in weight, e.g. in the case of titanium, removal of the water of hydration from the hydrated aluminum oxide, i.e., the water is known to come off at around 350.degree. F., which is the cure temperature of the polyimide, and the like.
Examples of the prior art relating to the present invention are set forth below.
U.S. Pat. Nos. 3,356,525 and 3,644,135 are directed to coating or impregnating carbon fibers or cloth with various metal carbides obtained by reacting carbon-containing materials or polymers with various metals.
U.S. Pat. Nos. 3,523,056 and 3,604,257 are directed to the production of glass cloth impregnated with a polyimide resin.
U.S. Pat. No. 3,642,681 discloses a coating composition containing a polysilicic acid component.
A number of patents including, for example U.S. Pat. No. 3,079,273, disclose formation of various silicon carbide articles.
Accordingly, one object of the invention is to provide resin compositions and composites having high fire resistance and low thermal conductivity. Another object is the provision of compositions and composites of the above type having utility as a fire barrier, and particularly applicable as an acoustic panel-fire wall structure in aircraft, capable of withstanding high temperature, e.g. a 2,000.degree. F. flame temperature. A still further object is the provision of resin compositions, particularly polyimide compositions and composites, such as polyimide-glass fabric composites or laminates, having incorporated therein a substance which substantially increases the fire resistance of the resin and reduces its thermal conductivity, and functions to stabilize the resin or resin char, at high temperatures, e.g. a 2,000.degree. F. flame temperature.